An X-ray CT apparatus is an apparatus which applies X-rays from the periphery of an object, collects data regarding the intensity of X-rays transmitted through the object by using an X-ray detector, and generates, as an image, information regarding a distribution of an X-ray absorption coefficient of the inside of the object on the basis of the collected data. The intensity or ray quality (spectral distribution) of X-rays radiated from an X-ray source of the X-ray CT apparatus temporally changes. In order to compensate for the temporal change, reference correction is performed on data measured by the X-ray detector.
In the reference correction, generally, one to a plurality of detection element channels at both ends of the X-ray detector are used as calibration detection elements (hereinafter, referred to as reference detectors), and X-rays which are not transmitted through an object are directly detected by the reference detectors. An image processing device (data processing device) of the X-ray CT apparatus calibrates X-ray intensity levels obtained by other channels with an X-ray intensity detected by the reference detectors as a reference, and generates an image such as a tomographic image by using a calibrated signal.
However, for example, in a case where a physique of an object is large, or a case where an object is disposed to be deviated from the rotation center of a scanner, a normal reference signal may not be obtained since X-rays incident to the reference detectors are blocked by the object. In order to solve such a problem caused by protrusion of an object, signals obtained from the reference detectors at both ends are compared with a predetermined threshold value, and it is determined whether or not the reference detectors are shielded on the basis of the magnitude of a signal intensity. In a case where one of the reference detectors is shielded, an artifact is prevented from becoming apparent by using only a signal from the other reference detector as reference correction data.
PTL 1 discloses a method in which output data from each of reference detectors at both ends is monitored in each measurement view, the maximum signal value up to the previous view of a view which is a calculation target is held, protrusion is determined in a case where the output data is less than a predetermined threshold value, and measured data in which protrusion occurs is replaced in the maximum signal value up to views before the protrusion occurs. According to this method, even in a case where reference channels at both ends are shielded, reference correction is appropriately performed.
Meanwhile, in recent years, it has been frequently performed to scan a relatively large object as exemplified by metabolic syndrome. There is a bed which is horizontally movable in a state in which an object is placed on the bed, and the bed may protrude out of a field of view (FOV). From this background, it is necessary to correct protrusion with high accuracy even in a case where protrusion occurs in most of the measurement views during scanning. In order to improve the accuracy of reference correction, it is preferable to perform correction by using an X-ray output change component in real time during main scanning.